i. Technical Field
The present invention pertains to improvements in pulp screening apparatus, and pertains particularly to an improved screen basket for use in pressure screens which has increased wear life, as compared to similar screen baskets known heretofore.
ii. Prior Art
Screens are used to separate acceptable papermaking fiber from unacceptable constituents of a slurry of pulp fiber in preparing the wood fiber for the papermaking process. In typical wood pulp screens, a slurry of pulp flows through a perforate cylindrical screen plate or basket which may be smooth, or which may have a contoured surface facing toward the stock flowing through the screen, to increase the effective screening area. It is known to utilize different hole, slot or hole and slot combinations for optimizing screening performance. To aid in passing the acceptable pulp through the screen plate, and to reduce plugging, pulsations are generated in the slurry, such as by passing a hydrofoil-shaped member past the screen plate. It is also known to use blunt, turbulence generating members on a screen rotor, to agitate the stock along the screen surface and improve screening efficiency. U.S. Pat. No. 4,855,038 discloses a pulp screen rotor which generates both turbulence and pulse for improved screening efficiency
To provide sufficient strength to the screen plate, which generally is a basket-shaped member surrounding the rotor, to withstand the pressures experienced in the pressurized screen, and to increase the screening capacity by presenting increased screening area, it has been the accepted practice to provide a thickly-walled screen plate which is machined to present the desired surface. Such machining is a time-consuming and expensive process. As a result of the manufacturing restrictions in the machining process, which are imposed at least in part by the machine tools themselves, the total available open accepts flow area in the screen plate has been limited, and the final shape of the screen plate has been a compromise between the limitations of machining and the desired optimum screening shape.
Since the machined screen plates are a single, integral piece, wear or damage in a small area has required replacement of the entire screen plate; including the screening surfaces, mounting surfaces, and support members. When the wear or damage occurs prematurely, the expense of operating such a screen increases significantly.
In co-pending application U.S. Ser. No. 07/206,151, filed Jun. 10, 1988, now U.S. Pat. No. 4,954,249 a modular screen basket is disclosed wherein relatively thin sheet-like material is formed into the desired shape or contour. Stamping, pressing, and bending techniques are taught to form the screen plate, which does not require machining. The holes or slots are taught to be created during, before, or after formation of the undulations in the sheet-like material, with suggested forming techniques including laser beam cutting and punching. Utilizing a punch die as part of the forming process is suggested as a simplified, cost-effective manner for forming the openings.
While laser beam cutting as disclosed in U.S. Ser. No. 07/206,151 made possible the formation of slots of various configurations heretofore unavailable through machining, in the use of baskets having slots formed in that manner, it was found that laser cutting left a rough edge on the cut line, which promoted basket plugging. Chemical polishing of the cut surface was not totally satisfactory in solving the problem. Also, laser cutting with the chemical polishing proved to be quite expensive.
Lancing the thin sheet material used for the baskets is a more economical process, but was found to result in the formation of tiny cracks at the end of the lanced slot. During fatigue testing, the minute cracks formed during lancing propagated, potentially causing premature basket failure. Modification of the lancing tool, including the provision of radii at each end of the lanced slots, and numerous attempted slot configuration modifications proved unsatisfactory in controlling crack propagation.